This thesis deals with the explanation and compensation of the effects ‘decay’ and ‘snapback’ in superconducting accelerator magnets, in particular in those used in the new Large Hardron Collider at CERN. During periods of constant magnet excitation, as for example during the injection of particles in the storage ring, the magnetic field in superconducting accelerator magnets shows a decay behavior. As soon as the particles are accelerated, the magnets are ramped, and the magnetic field ‘snaps back’ to the original hysteresis curve. Decay and snapback affect the beam in the machine and have to be compensated precisely in order to avoid losses of particles.

Decay and snapback in superconducting accelerator magnets

The quality of the magnetic field in superconducting accelerator magnets is associated with the properties of the superconducting cable. Current imbalances due to coupling currents /spl Delta/I, as large as 100 A, are induced by spatial variations of the field sweep rate and contact resistances. During injection at a constant field all magnetic field components show a decay behavior. The decay is caused by a diffusion of coupling currents into the whole magnet. This results in a redistribution of the transport current among the strands and causes a demagnetization of the superconducting cable. As soon as the field is ramped up again after the end of injection, the magnetization rapidly recovers from the decay and follows the course of the original hysteresis curve. In order to clarify the interactions between the changes in current and magnetization during injection the authors performed a number of experiments. A magnetic field with a spatially periodic pattern was applied to a superconducting wire in order to simulate the coupling behavior in a magnet. This model system was placed into a stand for magnetization measurements and the influence of different powering conditions was analyzed.

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Interaction between current imbalance and magnetization in LHC cables

The selection and use of reference magnets as a mean to improve predictability and reproducibility of the operation of the LHC depends on whether few magnets can be found that represent the average behavior of a sector, or, in the limit, of the whole accelerator. The purpose of this note is to show that it is possible to select magnets that represent well, on average, a family in the population of the magnets produced so far. We refer to this property by saying that the behavior of a single magnet can be scaled to be equal to the average of a family of magnets. The procedure used to achieve this equivalence is referred to as the scaling law.

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Scaling laws for the field quality at injection in the LHC dipoles

The problem of estimating the random errors in the LHC dipole is considered. The main contributions to random errors are due to random displacements of the coil position with respect to nominal design and to the variation of the magnetization of the superconducting cable. Coil displacement can be induced either by mechanical tolerances or by the manufacturing process. Analytical and numerical scaling laws that provide the dependence of the random errors due to random displacements on the multipolar order are worked out. Both simplified and more realistic models of the coil structure are analysed. The obtained scaling laws are used to extract from experimental field shape data the amplitude of the coil displacements in the magnet prototypes. Finally, random errors due to interstrand resistance variation during the ramp are estimated.

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M. Schneider

Papers

Decay and snapback studies on the LHC dipole model magnets : a scaling law